Secondary batteries are widely used in recent years as a power source of portable devices. In particular, lithium ion secondary batteries can be small in size and low in weight, have a high energy density, and can be repeatedly charged and discharged. On the basis of such properties, lithium ion secondary batteries are expected to grow in demand. Such lithium ion secondary batteries are utilized in apparatuses such as cellular phones and notebook personal computers, taking advantage of their high energy density. Since the apparatuses to which secondary batteries are applied have been improved in performance, secondary batteries are demanded to be further improved in performance. For example, secondary batteries are demanded to have improved performances such as an ability to maintain their capacity even after charge and discharge are repeated in a high-temperature environment (high-temperature cycle property).
A secondary battery is provided with a separator between a positive electrode and a negative electrode in order to improve performances. As the separator, there is known a porous membrane obtained by applying a layer of a slurry containing non-conductive particles onto a substrate and then drying the obtained layer. As such a slurry, a so-called aqueous slurry prepared by using water as a solvent is becoming widespread in order to, for example, reduce load on the environment (Patent Literature 1). Such a slurry can be prepared with physical properties such as viscosity falling within preferable ranges, whereby favorable application and facilitated manufacturing of a high-quality porous membrane can be performed. For adjusting such physical properties, a cellulose-based polymer such as carboxymethyl cellulose (CMC), for example, is often added to such a slurry.